Devices are needed for generating alternating voltages whose phases are shiftable relative to each other.
Alternating voltages which are continuously shiftable in phase by a constant angle independently of their frequency are needed to control machining operations on workpieces. If, for example, a workpiece is moved at a variable speed past several machining tools arranged in tandem in the direction of movement and the machining tools are to perform an operation at at least two points of the workpiece spaced a fixed distance independently of the rate of movement, means to control the tools are required. This can be achieved by controlling the tools by means of a.c. voltages shiftable in phase with respect to each other by a constant, frequency-independent angle.